1. Field of the Invention
The present invention is related to data bus transceivers for transmitting and receiving digital data on unbalanced data buses. More specifically, the present invention is related to transceivers having trapezoidal drivers and filtered differential receivers for reducing the effects of noise on a high-speed data bus.
2. Description of the Prior Art
Conventional computer systems use high-speed computer data buses employing Schottky bus transceivers for transferring digital data from one computer or peripheral device to another computer or peripheral device. These transceivers are designed to provide high-output currents for charging and discharging the relatively large bus capacitances quickly. These high-speed transitions are characterized by peak slew rates of approximately 5 volts per nanosecond. These high slew rates cause considerable noise to be capacitively coupled from one data line to adjacent data lines on the data bus. Additional noise may also be induced on the data lines by reflections and sources external to the data bus.
Conventional bus receivers are designed to respond to the high-speed transitions on the data bus and to provide for low propagation delays. Unfortunately, their fast response results in a high sensitivity to noise pulses on the data bus. The combined effect of the noise on the data buses and the sensitivity of the receivers to noise severely limits the total length of the data bus that can be used with acceptable reliability. For instance, typical minicomputer data buses are limited to bus lengths of ten feet or less by noise. It is desirable to extend the length of the data bus and still maintain high reliability in the data transferred.
Miller feedback integrators have been used to produce trapezoidal waveforms as drive waveforms resulting in a reduction in slew rates and the amount of noise coupled to adjacent lines. Such a circuit is taught in U.S. Pat. No. 3,453,452 entitled Trapezoidal-Waveform Drive Method and Apparatus issued to D. A. Starr, Jr. on July 1, 1969. However, Starr's circuit was directed towards a driver used in electrostatic multiple-electrode matrix printer apparatus of the type utilized for forming characters and symbols on record surfaces. Thus, Starr did not address the problems associated with receiving and reconstructing highspeed waveforms. One of the particular difficulties associated with the use of unbalanced trapezoidal drivers for high-speed data buses is that the trapezoidal pulses and the recovered digital waveforms must be of a high fidelity. Specifically, there must be a low distortion of the pulse width of transmitted pulses. Further, a transceiver must have a high degree of noise immunity and a minimum propagations delay. The present invention addresses the problems associated with transmitting and receiving high-speed waveforms on a data bus and reconstructing the original digital waveform quickly, with low distortion and with a high degree of noise immunity.